1. Field of the Invention
The present invention relates to multiple degree-of-freedom (DOF) parallel mechanisms which can provide quick and precise manipulative capabilities.
2. Description of the Related Art
The vast majority of multiple degree-of-freedom mechanisms that are used in robotic or teleoperator applications are so-called serial mechanisms. A serial mechanism is one in which a plurality of links are connected together in series to form an open chain and are moved with respect to each other by actuators connected between them to manipulate an object supported at the remote end of the chain of links. This type of mechanical mechanism has the advantages of the ability to access large workspaces, and of simplicity of design and geometric analysis. It has been shown that the forward kinematic problem is always directly solvable for serial mechanisms. The forward kinematic problem is defined as the task of solving for the position and orientation of the remote end of the mechanism on which a tool is mounted, given the lengths of all of the links and the angles between adjoining links.
Despite the above mentioned advantages, serial mechanisms are inherently plagued with a number of disadvantages. For one, the links at the base of a serial mechanism must support all of the more remote links of the mechanism. As a result, large actuators are required to drive the actuated joints at the base of the mechanism. For precise control, it is advantageous to have an actuator as close as possible to the tool or other object being driven by the actuator. With a serial mechanism, having an actuator close to the object being driven compromises the overall performance of the mechanical system, since actuators are typically heavy electric motors. In the case of a robotic wrist, for example, the designer must choose between locating the actuators that drive the robotic wrist directly at the wrist joints, and locating the wrist actuators towards the base of the robot and using a complex series of cables, gears, or other transmission devices to connect the wrist actuators to the wrist joints. The former choice allows precise control of the wrist but also requires that elbow and shoulder actuators located closer to the base support these wrist actuators, resulting in a large load being applied to the elbow and shoulder actuators. The latter choice reduces the moving mass which the elbow and shoulder actuators of the robot must support, but it also introduces numerous potential sources of error in the control of the position and/or force of the wrist, including backlash, friction, and wear. Another problem of serial mechanisms occurs when the position of the mechanism remote from a support structure is determined by sensors, such as encoders, which are located at the joints of the mechanism and measure the angles between adjoining links. Errors in measurement by the encoders are cumulative, i.e., the error in the calculated position of the remote end of the mechanism is a sum of errors of the individual encoders, so it is difficult to determine the position of the remote end with accuracy. Even when there is no encoder error, calculation of the position of the remote end may be inaccurate due to bending of the links forming the serial mechanism. These problems occur not just with robotic wrists but with serial mechanisms in general.
Another variety of multiple degree-of-freedom mechanism is referred to as a parallel mechanism. In parallel mechanisms, a plurality of actuators drive a tool or other object in "parallel", typically via a plurality of stiff links and joints. The term parallel in this sense means that the links share the load being supported by the mechanism, and it does not require that the links be geometrically parallel or imply that they are. Parallel mechanisms are inherently stiffer, quicker, more accurate, and capable of carrying higher loads than serial mechanisms. This is because parallel mechanisms have multiple mechanical tics between a base support structure and the object being supported so that the weight of the object is divided among a plurality of members, whereas in serial mechanisms, each link must support the entire weight of the object. A well-known example of a parallel mechanism is the Stewart Platform in which a load is supported by a plurality of links which can be adjusted in length by actuators to vary the position and orientation of the load. A parallel mechanism typically has all of its actuators mounted either on or relatively close to a base support structure, so the actuators either do not move or move very little during the operation of the mechanism. This minimizes the moving mass of the mechanism, making it much quicker than an equivalent serial mechanisms. Furthermore, since the entire load carried by the mechanism is not applied to each actuator as in a serial mechanism but is distributed among the actuators, the load capacity of the mechanism can be greatly increased relative to that of a serial mechanism without requiring large capacity (and thus bulky and heavy) actuators. In addition, errors in encoders or other sensors for sensing the position or orientation of the links forming a parallel mechanism are averaged rather than summed as in a serial mechanism, so the position and orientation of a load can be determined with high accuracy. A parallel mechanism is akin to a truss or space frame-type structure in which a load is supported by multiple paths to ground rather than by a single path. A mechanism is considered fully parallel if it has no actuators connected in series.
In spite of such advantages, parallel mechanisms have not achieved widespread acceptance as robotic or teleoperated devices due to a number of drawbacks. One is that conventional parallel mechanisms have limited reachable workspaces compared to serial mechanisms, so they are limited to tasks which do not require large workspaces. This is in part because parallel mechanisms have multiple mechanical ties to a fixed support structure whereas serial mechanisms have only one, and in part because the parallel links of a parallel mechanism can interfere with one another in certain positions. In addition, the forward kinematics problem for a parallel mechanism can be extremely complex mathematically, and in many cases it is not solvable, often making real time control of a parallel mechanism difficult or impossible.
Aside from the above problems, both parallel and serial mechanisms of conventional design tend to suffer from backlash in the components, relatively high friction, a narrow operational bandwidth, and high inertia which make high positional resolution and highly sensitive force control difficult to achieve.